The present invention relates to a scattered light smoke detector with an optoelectronic arrangement for measurement of scatter signals below a forward and a backscatter angle, and with evaluation electronics for obtaining a measured value from the scatter signals and comparing an alarm value derived from this signal with an alarm threshold.
It has long been known that with forward scatter and backscatter the two scattered light components differ in a characteristic manner for different types of fire. This phenomenon is described for example in WO-A-84/01950 (=U.S. Pat. No. 4,642,471), in which one of the disclosures is that for different types of smoke a different ratio of the scattering at a small scatter angle to the scattering at a large scatter angle can be utilized for detection of the smoke type. The larger scatter angle could also be selected as greater than 90°, meaning evaluation of the forward scatter and backscatter.
For a scattered light smoke detector described in EP-A-1 022 700 (=U.S. Pat. No. 6,218,950) of the type mentioned above a light/dark quotient which can be utilized for detection of the smoke type is calculated from the scatter signals. The two scatter signals are summed and the total is multiplied by the given light/dark quotient. The measured value is thus weighted depending on the ratio of the scatter signals, in which the scatter signal of a dark aerosol is subject to a higher weighting than the scatter signal of a light aerosol.